1. Field of the Invention
The present invention relates to a technique for removing a resist and, more particularly, to a method for removing a resist coated on a layer, e.g., an insulation layer, formed on a semiconductor substrate, and a semiconductor device formed by using that method.
The resist removal technique according to the present invention is used, for example, when a resist used as a mask in an etching process or an ion implantation process employed in the production of a semiconductor integrated circuit is removed prior to a subsequent process.
2. Description of the Related Art
Conventionally, the removal of a resist is usually implemented by a chemical process such as a wet process, a process using an "afterglow" of plasma, a process using ozone, and the like. For example, the wet process is carried out by using a solution of an acid, an alkali, an organic solvent, and the like, e.g., a commercially available resist removal solution.
However, it is known that a resist cannot be perfectly removed by the chemical process. For example, in the ion implantation process, a resist is carbonized. Also, the surface of the resist is damaged in the etching process using fluorine gas or fluoride. The carbonized or damaged portion formed during these processes is not perfectly removed, and remains even after the resist removal process.
In view of the above circumstances, the chemical removal method has been replaced by a dry process or an ashing method, e.g., a plasma removal method using gas having a high oxygen content. This plasma removal method includes the processes wherein a gas is brought to a plasma state by applying a high frequency electric field, and the like thereto, generating a multiple of charged particles, e.g., electrons, ions, and the like therefrom, and removing a resist by utilizing a chemical reaction between the generated particles and the constituent atoms or molecules of the resist. The plasma removal method completely removes the resist, because the charged particles in the plasma have sufficient energy to break the bonds between the constituent atoms or molecules of the resist.
The accuracy in fabrication of semiconductor devices, or miniaturization or degree of high integration of semiconductor devices has recently been required, and accompanying this requirement, the thickness of an insulation layer formed on a semiconductor substrate, e.g., a silicon dioxide layer (SiO.sub.2 layer), has been reduced to less than 20 nm (200 .ANG.).
However, during the production of a semiconductor device having such a thin SiO.sub.2 layer formed on the surface thereof, the inventors removed a resist coated on the SiO.sub.2 layer by the plasma removal method, and found the following problems arising therefrom.
It is known that the resist contains a small quantity of heavy metal atoms. For example, a commercially available positive type resist (OFPR-800 produced by Tokyo Ohka, Co., Ltd.) contained 360 ppb of iron (Fe), 190 ppb of lead (Pb), 20 ppb of copper (Cu), and the like.
It was found that such heavy metal atoms sometimes permeated through the silicon dioxide layer having a thickness of less than 20 nm (200 .ANG.), during the plasma removal process, entered the semiconductor, e.g., silicon (Si) substrate, and remained within the semiconductor, resulting in the destruction of functional elements formed in the semiconductor. In particular, it was found that the minority carrier generation lifetime was reduced from 500-600 .mu.sec to 100 .mu.sec, depending upon the plasma condition such as plasma density, plasma temperature, gas pressure, and the like, the thickness of the silicon dioxide layer and the quality thereof, the substrate temperature, and the like.
It is also known that the resist further contains alkaline metal atoms of sodium (Na), potassium (K) and the like, together with the aforementioned heavy metal atoms, and that the alkaline metal atoms and the heavy metal atoms enter the semiconductor during the plasma removal process. In this case, it is possible to remove the alkaline metal atoms by a subsequent process, e.g., an Hcl gettering. However, the leavy metal atoms cannot be removed by any subsequent process.
Accordingly, it should be noted that this contamination in the semiconductor by alkaline metal atoms is not permanent but temporary, and accordingly, is essentially different from the contamination in the semiconductor caused by the aforementioned heavy metal atoms.